The following pertains to the pipe or tube inspection arts, pipe or tube maintenance arts, industrial systems maintenance arts, and related arts.
Pipes or tubes (these terms being used interchangeably herein, along with variants such as piping or tubing) carry fluids for diverse purposes. As some non-limiting illustrative examples, steam pipes carry steam, water pipes carry water (possibly with various additives or so forth), pipes or tubes associated with a nuclear reactor may carry steam or water having some level of radioactive contamination, or may carry deuterium (e.g., a pressurized heavy water reactor, PHWR), gas pipes may carry hydrocarbon-based fluids such as natural gas, or in another context may carry process gases such as nitrogen or oxygen, and so forth. An illustrative example of a PHWR is the Canadian Deuterium Uranium (CANDU®) reactor. Fluids carried by industrial piping may be at elevated temperature and/or pressure (or conversely low temperature, e.g., liquid nitrogen, and/or low pressure, e.g., vacuum piping), may be transported at high flow rates, or otherwise may introduce stress to the pipes. Fluids carried by industrial piping may also introduce chemical stress, e.g., corrosion—for example, some nuclear reactors employ coolant water containing boric acid which serves as a soluble neutron poison.
The various fluids carried by industrial piping can damage the piping by various mechanisms, including high flow velocity-related damage, chemical damage (e.g., in piping carrying corrosive fluid), radiation damage (e.g., in piping carrying fluid contaminated with radiation), or so forth. Damage tends to be more extensive at pipe bends and at welds between pipe segments. In view of these concerns, pipe inspection is common industrial practice, and may be mandated by applicable governmental regulations, for example in the nuclear industry.
However, pipe inspection is challenging due to the typically long lengths of piping that need to be inspected, and the need to inspect pipe bends, weld joints, or other features some of which may not be readily accessible. Additionally, industrial piping is sometimes arranged in a densely packed layout, with pipes sometimes passing within close proximity to one another, again limiting access for inspection.
A known approach for pipe inspection employs an inspection head that includes suitable sensing elements, such as an ultrasonic testing (UT) inspection head, radiographic inspection head, eddy current inspection head, or so forth, that is inserted into the pipe and pushed or drawn through the pipe using a snake, cable, or the like. These approaches require access to the pipe interior, and therefore cannot be used to inspect piping during operation (that is, when the piping is carrying working fluid). Another consideration is that the rotational orientation of the inspection head inside the pipe, as well as its position along the pipe, usually must be known or tracked as the inspection head moves through the interior of the pipe. This may be addressed by suitable spatial encoding of the inspection head position, for example based on the rotation of driving wheels of a robotic apparatus, but any slippage of the spatial encoding mechanism during the inspection can lead to spatial encoding errors.
An illustrative example of a difficult pipe inspection task is the inspection of feeder pipes in a CANDU® heavy water nuclear reactor. In this heavy water reactor the radioactive core is arranged as an array of mutually parallel horizontally oriented fuel tubes. Each fuel tube contains a fuel bundle comprising a fissile material such as uranium with low 235U enrichment (or no enrichment at all) or mixed oxide fuel (MOX fuel). To achieve critical mass for the nuclear chain reaction, the fuel tubes must be spaced closely together in a relatively tight array. Feeder pipes carry primary (deuterium) coolant to these closely spaced fuel tubes, and the density of feeder pipes near their connections with the fuel tubes is high, with feeder pipes passing within close proximity to one another and including various bends in order to fit all the feeder pipes into the limited available space. By way of illustrative example, some Candu® reactors include 480 fuel tubes fed by 480 inlet feeder pipes and 480 outlet feeder pipes. The feeder pipes are prone to corrosion over time due to the continual flow of radioactive deuterium coolant, especially at feeder bends and at pipe segment welds (although corrosion can occur elsewhere and the pipe inspection typically inspects both tight and large-radius bends as well as straight sections). A break in a feeder pipe due to such corrosion constitutes a loss of coolant accident (LOCA) producing a radiological release into the surrounding containment structure, and may require immediate shutdown of the nuclear reactor and extensive post-shutdown cleanup and incident analysis before the reactor can be brought back online. Consequently, governing nuclear regulations in the United States, Canada, and some other jurisdictions require periodic inspection of all feeder pipes to detect any thinned pipe regions. It is preferable to perform such inspections without accessing the interior of the pipe. For example, during a typical CANDU® feeder pipe inspection process, only one feeder pipe per reactor can be isolated with a liquid nitrogen freeze plug (as no valves are available on the feeder pipes), and drained at a time per the regulator authority. Such a process would also limit productivity if the inspection were done from the ID. Feeder pipe inspection is further complicated by high radioactivity levels in the vicinity of the reactor core which limits access to the feeder pipes by plant operators. In a typical inspection procedure, a technician approaches the reactor core in a radiation-shielded trolley or cart (RDP platform), and performs inspection operations through slits provided by panel shielding curtains. Even with these precautions, the technician's exposure time to radioactivity proximate to the reactor core limits the time for performing the inspection in accord with radiation exposure limits that apply to all nuclear plant operators.
A known approach for performing CANDU® reactor feeder pipe inspection uses an inspection ring that is driven along the outside diameter of the feeder pipe by a robotic crawler. However, this approach has been prone to slippage which introduces spatial encoding errors, and the robotic crawler can exhibit difficulty crawling over sharp pipe bends or welds that protrude from the pipe's exterior surface. Improvements might be obtained by using more complex robotic crawlers (e.g., a multiple-axis robotic arm), but at commensurate increase in robotics manufacturing cost as well as increased likelihood of breakdown in the field due to the increased robotic complexity, and possibly increased setup time leading to increased radiation exposure for the technician.
While CANDU® reactor feeder pipe inspection is described herein as an illustrative example of a difficult pipe inspection task, it will be appreciated that similar problems may arise in any piping inspection task in which the piping is to be inspected while in-service (or in which the pipe interior is otherwise inaccessible, for example due to corrosive residue deposits inside the pipe). There remains an unfulfilled need for improved pipe inspection apparatus of low cost and high reliability that can inspect in-service piping, maintain accurate position encoding in both axial and rotational orientations, and provide other benefits.